Flat negative electrodes for nickel metal hydride batteries are conventionally manufactured by the following process. An ingot is cast of the appropriate nickel hydride alloy. Powder is then produced from the ingot by one or more of grinding, crushing, milling or decrepitation. The powder is then mixed with a binder such as a suitable polymer material, and a conducting medium such as carbon. The mixture is then pressed or rolled onto a perforated nickel sheet and formed into a sandwich with a further such sheet.
Gas atomisation is an existing technique used in the production of powders. Variants of this technique for example are used for the production of coated strips or plate by spraying, whilst alleviating the requirement for rolling. This technique is used to provide a non-porous metal coating or solid ingot as near as possible to full density. The technique comprises melting the metal of the coating or ingot which is then passed through an orifice to form a stream. The stream is subsequently passed through a jet or jets of gas which impact the stream to provide atomisation thereof. The atomised particles are projected to deposit on a substrate or allowed to freeze in-flight into powder.
According to the present invention there is provided a method of forming an electrode, the method comprising forming a porous coating of a required alloy by gas atomisation on a substrate.
The porosity of the coating is preferably greater than 13%, and desirably greater than 50%. The substrate is preferably also porous.
The porosity of the coating is preferably provided by controlling one or more of: the height of deposition of the spray onto the substrate; the rate of deposition; the duration of deposition; the speed of the atomised particles impacting on the substrate; the size distribution of the spray droplets; the melt super heat; the temperature about melting point. the substrate temperature; the substrate thermal conductivity: the conductivity of the gas; the temperature of the gas.
The alloy preferably comprises a nickel hydride alloy, and desirably MMNiAlMnCo. The abbreviation xe2x80x9cMMxe2x80x9d is used herein to refer to a combination of rare earth metals known in the art as xe2x80x9cmisch metal.xe2x80x9d
The substrate is preferably metaalic, and may be of nickel or copper, and may be in the form of a solid sheet, perforated sheet or gauze.
The relative position of the substrate is preferably changed during deposition to provide a substantially even deposition thereon The substrate may be moved; and/or the direction of gas flow against the melted alloy may be moved; and/or the direction of flow of the alloy may be moved. The substrate is preferably turned over at least once to provide deposition on opposite sides thereof.
The method is preferably carried out in an inert gas atmosphere, such as argon.
A member is preferably provided to collect overspray, and said member may comprise a container locatable beneath the substrate.
The substrate is preferably rolled to a required thickness following spraying. The substrate may be subject to a heat treatment after spraying.
The invention also provides an electrode made by a method according to any of the preceding nine paragraphs.
The electrode is preferably a negative electrode, and desirably a battery electrode.